Investigations of weak and dilute magnetic behaviour in organic and inorganic systems

Muon spin relaxation is an ideal tool with which to study dilute magnetic systems, coupled with tailored bulk magnetic susceptibility measurements it is possible to examine previously unobserved magnetic exchange interactions. Investigations into non-stoichiometric LaCo03 reveals evidence of magnetic excitons in transition metal oxides for the first time. Moreover, data is presented that supports the concept of the defect driven excitons interacting with the stoichiometric LaCo03 which is known to undergo a thermally driven spin state transition. The data suggest the occurrence of more than one possible magnetic interaction of the excitons. Hole doped La1-xSrxCo03 is of interest as it is known to be magnetically and electronically phase separated; by a direct analogy with magnetic excitons it is suggested that the Sr rich ferromagnetic clusters interact with the pure LaCo03 below the metal insulator transition (x = 0.18 ) . It is suggested that it is this interaction observed for the first time that enables the rich phase diagram of La1_xSrxCo03 . The persistent photoconductivity effect on the spin glass transition in the elilute magnetic semiconductor Ccl0.85Mn0.15 Te:In has been investigated using low temperature magnetic susceptibility measurements and for the first time muon spectroscopy. Muon measurements on an Al eloped sample clearly show the spin glass transition, however the presence of the DX centre, which causes PPC when doping with In donors, perturbs the muon response. Particular attention is paid to possibility of the DX centre trapping muonium and preventing the detection of the spin glass transition. PPC does not induce a change in the muon response, however continuous illumination of the sample allows the observation of the spin glass transition, suggesting the presence of multiple DX centres, moreover the centre is found to be diamagnetic. The search for magnetic ordering at room temperature in an organic material has generally neglected polymers. PANiCNQ combines a fully conjugated nitrogen containing backbone with molecular charge transfer side groups. This combination gives rise to a stable polymer with a high density of localised spins, which are expected to give rise to coupling. Magnetic measurements suggest that the polymer is ferri- or ferro- magnetic with a Curie temperature of over 350 K, and a maximum saturation magnetization of 0.1 JT-1 kg-1 . Magnetic force microscopy images support this picture of room temperature magnetic order by providing evidence for domain wall formation and motion

RealSense Relay Board

RealSense is an emerging technology owned by Intel. RealSense cameras are unique in the sense that they have depth sensing abilities allowing for object tracking and identification. Due to the wide range of applications RealSense cameras are found in a multitude of different environments. As with any new technology these cameras experience technical issues that can affect performance. It was the goal of this project to solve the particular problem of RealSense cameras freezing. RealSense cameras are data collection devices, often times the continuity of that data is extremely important and any downtime is to be avoided. Physical resets require someone on scene to unplug and plug back in each failing camera. This can be time consuming and ultimately costly. Often the data collected by the camera is operated on in a location thousands of miles from the actual physical location of the camera. In order to solve this problem we designed a interface Printed Circuit Board (PCB) that can be controlled via secure shell (SSH) from anywhere in the world and switch the power to the camera using a metal-oxide-semiconductor field-effect transistor (MOSFET). This particular board has a capacity of five RealSense cameras operating independently. After considerable design and manufacturing the boards did not pass the functionality test. Power control performs as intended, however, the SuperSpeed signal lines that pass the data from the RealSense through the PCB did not function as intended causing the cameras themselves to not function

New capabilities at pulsed muon facilities with pulsed radio-frequency techniques

At a pulsed muon source, a radio-frequency pulse timed to rotate the implanted muon spin polarization by 90° can be used to completely remove the frequency limitations imposed on conventional spin rotation experiments by the finite muon pulse width. This contribution presents two applications of the method, where controlling the excitation bandwidth of the radio-frequency pulse provides unique information. In the first case, an intense 90° pulse is used for broadband excitation of a paramagnetic system, while in the second case a low power pulse applied in the presence of a field gradient excites a limited frequency band leading to spatial localization of the muon signal

μSR study of stoichiometric NbFe2

The magnetic ground state of nominally stoichiometric single crystalline NbFe2 is investigated by bulk magnetisation and muon spin relaxation techniques. Magnetic order clearly emerges below the critical temperature TN=10.3 K and is dominated by randomly orientated quasi-static moments. The local field distribution observed by muons can be explained by the phenomenological Gaussian-broadened-Gaussian Kubo Toyabe relaxation function. The observed short range order could be used to describe a new magnetic ground state, but a helical spin density wave with an incommensurate amplitude modulation cannot be ruled out. The sensitivity of μSR to the local magnetic field distribution in the vicinity of the quantum critical point (QCP) in NbFe2 is clearly demonstrated via comparison with already published work. This suggests detailed measurements of the muon relaxation as the QCP is approached will reveal further details of the field distribution and fluctuations in Nb1−yFe2+yNb1−yFe2+y

Vacancies, disorder-induced smearing of the electronic structure, and its implications for the superconductivity of anti-perovskite MgC0.93_{0.93}Ni2.85_{2.85}

The anti-perovskite superconductor MgC$_{0.93}$Ni$_{2.85}$ was studied using high-resolution x-ray Compton scattering combined with electronic structure calculations. Compton scattering measurements were used to determine experimentally a Fermi surface that showed good agreement with that of our supercell calculations, establishing the presence of the predicted hole and electron Fermi surface sheets. Our calculations indicate that the Fermi surface is smeared by the disorder due to the presence of vacancies on the C and Ni sites, but does not drastically change shape. The 20\% reduction in the Fermi level density-of-states would lead to a significant ($\sim 70\%$) suppression of the superconducting $T_c$ for pair-forming electron-phonon coupling. However, we ascribe the observed much smaller $T_c$ reduction at our composition (compared to the stoichiometric compound) to the suppression of pair-breaking spin fluctuations.Comment: 11 pages, 3 figure

Bimetallic synergy enables silole insertion into THF and the synthesis of Erbium single‐molecule magnets

The potassium silole K2[SiC4‐2,5‐(SiMe3)2‐3,4‐Ph2] reacts with [M(η8‐COT)(THF)4][BPh4] (M=Er, Y; COT=cyclo‐octatetraenyl) in THF to give products that feature unprecedented insertion of the nucleophilic silicon centre into a carbon‐oxygen bond of THF. The structure of the major product, [(μ‐η8 : η8‐COT)M(μ‐L1)K]∞ (1M), consists of polymeric chains of sandwich complexes, where the spiro‐bicyclic silapyran ligand [C4H8OSiC4(SiMe3)2Ph2]2− (L1) coordinates to potassium via the oxygen. The minor product [(μ‐η8 : η8‐COT)M(μ‐L1)K(THF)]2 (2M) features coordination of the silapyran to the rare‐earth metal. In forming 1M and 2M, silole insertion into THF only occurs in the presence of potassium and the rare‐earth metal, highlighting the importance of bimetallic synergy. The lower nucleophilicity of germanium(II) leads to contrasting reactivity of the potassium germole K2[GeC4‐2,5‐(SiMe3)2‐3,4‐Me2] towards [M(η8‐COT)(THF)4][BPh4], with intact transfer of the germole occurring to give the coordination polymers [{η5‐GeC4(SiMe3)2Me2}M(η8‐COT)K]∞ (3M). Despite the differences in reactivity induced by the group 14 heteroatom, the single‐molecule magnet properties of 1Er, 2Er and 3Er are similar, with thermally activated relaxation occurring via the first‐excited Kramers doublet, subject to effective energy barriers of 122, 80 and 91 cm−1, respectively. Compound 1Er is also analysed by high‐frequency dynamic magnetic susceptibility measurements up to 106 Hz

Identification of oxidation state +1 in a molecular uranium complex

The concept of oxidation state plays a fundamentally important role in defining the chemistry of the elements. In the f block of the periodic table, well-known oxidation states in compounds of the lanthanides include 0, +2, +3 and +4, and oxidation states for the actinides range from +7 to +2. Oxidation state +1 is conspicuous by its absence from the f-block elements. Here we show that the uranium(II) metallocene [U(η5-C5iPr5)2] and the uranium(III) metallocene [IU(η5-C5iPr5)2] can be reduced by potassium graphite in the presence of 2.2.2-cryptand to the uranium(I) metallocene [U(η5-C5iPr5)2]- (1) (C5iPr5 = pentaisopropylcyclopentadienyl) as the salt of [K(2.2.2-cryptand)]+. An X-ray crystallographic study revealed that 1 has a bent metallocene structure, and theoretical studies and magnetic measurements confirmed that the electronic ground state of uranium(I) adopts a 5f3(7s/6dz2)1(6dx2-y2/6dxy)1 configuration. The metal-ligand bonding in 1 consists of contributions from uranium 5f, 6d, and 7s orbitals, with the 6d orbitals engaging in weak but non-negligible covalent interactions. Identification of the oxidation state +1 for uranium expands the range of isolable oxidation states for the f-block elements and potentially signposts a synthetic route to this elusive species for other actinides and the lanthanides

Magnetic frustration, short-range correlations and the role of the paramagnetic Fermi surface of PdCrO₂

Frustrated interactions exist throughout nature, with examples ranging from protein folding through to frustrated magnetic interactions. Whilst magnetic frustration is observed in numerous electrically insulating systems, in metals it is a rare phenomenon. The interplay of itinerant conduction electrons mediating interactions between localised magnetic moments with strong spin-orbit coupling is likely fundamental to these systems. Therefore, knowledge of the precise shape and topology of the Fermi surface is important in any explanation of the magnetic behaviour. PdCrO2, a frustrated metallic magnet, offers the opportunity to examine the relationship between magnetic frustration, short-range magnetic order and Fermi surface topology. By mapping the short-range order in reciprocal space and experimentally determining the electronic structure, we have identified the dual role played by the Cr electrons in which the itinerant ones on the nested paramagnetic Fermi surface mediate the frustrated magnetic interactions between local moments

Proximity-Induced Odd-Frequency Superconductivity in a Topological Insulator

At an interface between a topological insulator (TI) and a conventional superconductor (SC), superconductivity has been predicted to change dramatically and exhibit novel correlations. In particular, the induced superconductivity by an $s$-wave SC in a TI can develop an order parameter with a $p$-wave component. Here we present experimental evidence for an unexpected proximity-induced novel superconducting state in a thin layer of the prototypical TI, Bi$_2$Se$_3$, proximity coupled to Nb. From depth-resolved magnetic field measurements below the superconducting transition temperature of Nb, we observe a local enhancement of the magnetic field in Bi$_2$Se$_3$ that exceeds the externally applied field, thus supporting the existence of an intrinsic paramagnetic Meissner effect arising from an odd-frequency superconducting state. Our experimental results are complemented by theoretical calculations supporting the appearance of such a component at the interface which extends into the TI. This state is topologically distinct from the conventional Bardeen-Cooper-Schrieffer state it originates from. To the best of our knowledge, these findings present a first observation of bulk odd-frequency superconductivity in a TI. We thus reaffirm the potential of the TI-SC interface as a versatile platform to produce novel superconducting states.Comment: Accepted version for publication in Physical Review Letter

Triply bonded pancake π-dimers stabilized by tetravalent actinides

Aromatic π-stacking is a weakly attractive, noncovalent interaction often found in biological macromolecules and synthetic supramolecular chemistry. The weak nondirectional nature of π-stacking can present challenges in the design of materials owing to their weak, nondirectional nature. However, when aromatic π-systems contain an unpaired electron, stronger attraction involving face-to-face π-orbital overlap is possible, resulting in covalent so-called “pancake” bonds. Two-electron, multicenter single pancake bonds are well known, whereas four-electron double pancake bonds are rare. Higher-order pancake bonds have been predicted, but experimental systems are unknown. Here, we show that six-electron triple pancake bonds can be synthesized by a 3-fold reduction of hexaazatrinaphthylene (HAN) and subsequent stacking of the [HAN]3– triradicals. Our analysis reveals a multicenter covalent triple pancake bond consisting of a σ-orbital and two equivalent π-orbitals. An electrostatic stabilizing role is established for the tetravalent thorium and uranium ions in these systems. We also show that the electronic absorption spectrum of the triple pancake bonds closely matches computational predictions, providing experimental verification of these unique interactions. The discovery of conductivity in thin films of triply bonded π-dimers presents new opportunities for the discovery of single-component molecular conductors and other spin-based molecular materials